CMOS image sensors are attractive for use in a wide range of applications, such as still photography and video imaging products, due to their compatibility with VLSI circuit design and fabrication processing. Since low-cost, large-scale CMOS design and fabrication technologies that have been developed for large-volume VLSI circuits can be directly employed in the production of CMOS imagers, CMOS imagers are, in general, much more cost effective than imagers produced based on CCD technologies. As a result, for many applications, and particularly for consumer products, a CMOS imager is preferred over a corresponding CCD imager.
In comparing the performance of CMOS and CCD imagers, it is found that CMOS imagers are typically characterized as introducing into an imaged scene a level of fixed-pattern noise that is higher than that introduced by CCD imagers. Fixed-pattern noise in an image is here meant to refer to image noise that is substantially constant over time; i.e., fixed pattern noise does not substantially vary from frame to frame in a sequence of images, and any variation of the noise over time due to, e.g., temperature and aging effects, is so slow as to be undetectable in a sequence of image frames.
CMOS fixed-pattern noise is generally due to mismatches in the threshold voltages of the MOS transistors of a CMOS pixel, as well as being due to feedthrough of electronic charge, associated with the MOS pixel transistors, to the output of a pixel. Such threshold voltage mismatch and charge feedthrough effects can vary from pixel to pixel in a CMOS imager array. As a result, a pattern of noise is produced across an image, corresponding to the spatial variation of noise levels from pixel to pixel across the pixel array. A grainy image is the typical manifestation of the noise.
Beyond fixed pattern noise, charge feedthrough across CMOS pixel transistors is also found to produce a fixed offset in the lowest intensity limits of a CMOS imager. This fixed offset, also known as a dark offset, is generally constant across a CMOS pixel array. The dark offset reduces the available voltage swing of pixels, thereby limiting the dynamic range of the imager. Double-sampling of a CMOS pixel output, i.e., sampling of the pixel output at different times, e.g., during a pixel reset period and then again during an integration period, is a well-known technique often employed for correcting for the fixed pattern noise caused by MOS transistor threshold voltage mismatch. This technique enables a determination of the extent of a pixel output that is due to fixed pattern noise, in the manner described below. This double-sampling technique suffers, however, in that it does not enable the determination and/or correction of feedthrough of electronic charge across the pixel MOS transistors to the pixel output.
One method known for partial compensation of the impact of charge feedthrough is a calibration technique in which the dark offset and fixed pattern noise of an imager are determined under dark conditions, at a selected temperature, with a corresponding compensation factor then being applied to images once they have been collected. Although this technique does enable some amount of correction of the dark offset and fixed pattern noise caused by charge feedthrough, it requires the application of a calibration factor to the post-processing of images, typically by the end user of the imager.
Moreover, due to the calibration factor application's complexity, the calibration factor application is typically enabled only in very high-end imager designs and is not practical for general commercial imagers. But even when a calibration factor is employed, the calibration factor is determined only at one selected operating temperature at a single point in time; and therefore, the calibration factor does not account for temperature-dependent changes in dark offset and/or fixed pattern noise and further does not account for aging effects on the imager.
In summary, the double-sampling and dark offset compensation techniques conventionally employed in CMOS image processing are not fully effective and not optimally efficient in eliminating the effect of charge feedthrough on pixel output measurements.
Therefore, it is desirable to provide a non-complex process for compensating for both CMOS fixed pattern noise and electronic charge feedthrough across CMOS pixel transistors. Furthermore, it is desirable to provide a non-complex compensation process that accounts for temperature-dependent changes in dark offset and/or fixed pattern noise and further accounts for aging effects on the imager.